Conventional solar cell elements are configured to include a silicon substrate, thin wire electrodes that collect photogenerated carriers generated in a photoelectric conversion unit region of the silicon substrate, and light-receiving-surface bus electrodes (light-receiving-surface electrodes for connecting wiring members) that are connected to the thin wire electrodes and are for transferring collected photogenerated carriers to the output wiring members.
The output wiring member is a strip-shaped elongated copper foil made of copper (Cu) and the light-receiving-surface bus electrode (light-receiving-surface electrode for connecting the wiring member) is an electrode for bonding the wiring member made of copper foil and is formed to intersect with the thin wire electrodes. The light-receiving-surface bus electrode is formed by firing conductive paste containing particles of a highly conductive material, such as silver (Ag), as a filler with glass or resin as a binder in a similar manner to the thin wire electrode. Normally, as described in Patent Literature 1 below, the light-receiving-surface bus electrode and the wiring member are bonded with solder. Sn-based Pb-free solder, such as Sn-3Ag-0.5Cu (melting point is 218° C.), or Pb—Sn solder (melting point is 183° C.) is used as solder.
In such a method of bonding the wiring member by using the light-receiving-surface bus electrode formed on the silicon substrate in advance, there are two problems. Firstly, silver (Ag) used as a material of the light-receiving-surface bus electrode is expensive and a large amount of silver is needed to form the light-receiving-surface bus electrode with a thickness of 1 to 2 mm. Accordingly, it leads to an increase in manufacturing cost. Secondly, because the wiring member is bonded along the entire length of the solar cell, the solar cell element can be warped and damaged in some cases due to differential thermal expansion between the silicon of the solar cell element and the copper of the wiring member.
Therefore, in order to solve the above problems, conventionally, as described in Patent Literature 2 below, a method is proposed in which the wiring member is directly bonded to the light receiving surface of the solar cell element with thermosetting adhesive without using the light-receiving-surface bus electrode. Specifically, in this method, thermosetting resin is arranged to intersect with the thin wire electrodes of the solar cell, the wiring members are arranged thereon, and the wiring members are connected by thermally curing the adhesive in a state where the wiring members are pressed against the adhesive. The thin wire electrodes and the wiring members are electrically connected by coming into contact with each other and the force of maintaining the contact is exerted by the thermosetting resin. With this configuration, photogenerated carriers collected in the thin wire electrodes directly flow to the wiring members. Because the light-receiving-surface bus electrode is not formed, the solar cell module can be manufactured at low cost. Moreover, because the wiring members are bonded to the silicon substrate with resin having a Young's modulus that is approximately 1/10 of that of solder, warpage of the solar cell element can be reduced.